This invention relates to Sialon-base ceramic materials for tools, of which surfaces are coated with a hard layer, and more particularly to surface coated Sialon-base ceramic materials for tools which exhibit very excellent wear resistance when used in cutting tools and other wear-resisting tools.
In recent years, silicon nitride-base ceramic materials have become used in cutting tools and other wear-resisting tools. However, such ceramic materials suffer from low sinterability since silicon nitride (hereinafter referred to as "Si.sub.3 N.sub.4 ") is a compound having a strong covalent bond. Therefore, many ceramic materials of this type have conventionally been produced by means of hot pressing. According to the conventional hot pressing method, however, it is difficult to obtain products having complicated shapes from Si.sub.3 N.sub.4 -base ceramic materials, and the method suffers from low yield.
Attempts have been made to employ in cutting tools and other wear-resisting tools Sialon-base ceramic materials, which have higher sinterability than Si.sub.3 N.sub.4 and also have high thermal shock resistance as well as oxidation resistance. Such Sialon-base ceramic materials are composed mainly of a compound obtained by substituting Al and O, respectively, for part of Si and part of N in the crystal lattice of .beta.-Si.sub.3 N.sub.4, that is, .beta.-Sialon expressed by a chemical formula Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where z is larger than 0 but not more than 4.3. Such Sialon-base ceramic materials have high toughness but do not possess sufficient hardness, that is, their hardness is e.g. of the order of 92 in terms of Rockwell hardness (A scale). Therefore, such Sialon-base ceramic materials cannot show satisfactory wear resistance when used in cutting tools and other wear-resisting tools.